A steam tube dryer (hereinafter, appropriately called STD as well) being an indirectly heating rotary dryer is provided with a rotating shell of which length is 10 to 30 meters. Drying is performed in the rotating shell with heated steam as external heat for drying during a course where material to be dried, fed from one end side of the rotating shell is discharged from the other end side while the rotating shell is rotated.
Specifically, wet powders or granular powders being material to be dried are dried as being contacted to heated tubes in which steam and the like is fed as a heat medium, and concurrently, the dried material is sequentially moved to a discharge opening owing to rotation of the rotating shell. In this manner, the material to be dried is continuously dried.
Such an indirectly heating rotary dryer can be increased in size and is less expensive than an indirectly heating type disc dryer. In addition, drive operation is easy with less maintenance spots and required power is small. Accordingly, such an indirectly heating rotary dryer has been conventionally used in various fields as an apparatus to dry or cool material to be processed.
In an indirectly heating rotary dryer of the related art illustrated in FIG. 11, a plurality of heating tubes 111 is arranged at the inside of a rotating shell 110 as being in parallel to an shaft center of the rotating shell.
However, an upper limit value of a hold up ratio ((volume of material to be dried retained in the rotating shell)/(inner volume of the rotating shell)) of material H to be dried in the rotating shell is approximately 30% owing to a factor of a position through which the material H to be dried is fed. Accordingly, there are not many heating tubes 111A, which contribute to heating as being contacted to the material H to be dried. The ratio of the heating tubes 111A, which contribute to heating, is on the order of 30% with respect to the total heating tubes 111.
Consequently, the heating tubes 111 have not been effectively utilized in a conventional apparatus owing to existence of the heating tubes 111B, which are not contacted to the material H to be dried, or short contact time of the heating tubes being close to a shaft center of the rotating shell even though they are heating tubes 111A, which are contacted to the material.
Further, since the upper limit value of the hold up ratio of material to be dried is approximately 30% as described above, the heating tubes are rarely contacted to the material to be dried even when being arranged in the vicinity of the center in the rotating shell. Accordingly, in the conventional apparatus, heating tubes are not arranged in the vicinity of the shaft center of the rotating shell, thereby resulting in being inefficient and non-economical.
On the other hand, it has been evaluated to increase the hold up ratio of material to be dried in order to increase a contact area between the material to be dried and the heating tubes. However, this case results in causing a power increase for lifting the material to be dried within the rotating shell. Accordingly, the above has been also non-economical with low energy efficiency.